This project is designed to test the hypothesis that serum growth factors are involved in the control of catch-up growth. The investigation will be carried out in rats undergoing growth arrest and recovery after treatment with fasting, cortisone injections, or propylthiouracil feeding. Determinations will be made of plasma growth hormone (GH) by radioimmunoassay, somatomedin (SM) by radiobioassay, pituitary weight, and pituitary GH content. SM will be assayed in rat and porcine rib cartilage systems. Metabolic clearance rate and secretory rate of GH will be determined during the same experimental periods. Sulfate uptake by treated rat rib cartilage during incubation in vitro with normal rat serum will be measured in order to determine the ability of treated cartilage to respond to SM during growth arrest and the recovery period. After parabiosis of normal to normal and normal to cortisone measurements will be made of SM, skeletal and soft tissue growth, and sulfation of rib cartilage in vitro in order to determine whether humoral factor(s) other than SM may affect the recovery of cartilage. Light and electron microscopy will be carried out in representative animals by a collaborator of the investigator in order to correlate structure and metabolic findings whenever possible. These data are expected to determine how SM, GH and cartilage interact during growth arrest and recovery.